The invention pertains generally to the measurement of the mass airflow ingested into an engine having a fuel injection system and is more particularly directed to vane-type airflow sensors for such measurements.
Automotive fuel injection systems are becoming familiar to many of the driving public today. These systems include an electronic control which regulates the air/fuel ratio of the combustion process with regard to a schedule based on the operating parameters of the engine. Generally, the mass fuel flow of an injection system is controlled with high precision by measuring the mass airflow injected into the induction tube of the engine and thereafter dividing the scheduled air/fuel ratio by the measured amount. The calculated fuel amount is then injected by conventional solenoid fuel injectors at predetermined times into the engine. However, the accuracy of the entire control process depends heavily on the precision of the mass airflow measurement.
The mass airflow inducted into an engine can be measured indirectly as has been previously accomplished in speed-density systems. In the indirect system the engine is envisioned as a constant volume pump wherein the volume of airflow is directly proportional to speed. This volume calculation is transformed into a mass airflow measurement by modifying the volume resultant for density changes in the airflow due to manifold pressure and the ambient temperature. This measurement technique has the drawback that it must be further compensated for the volumetric efficiency of the particular engine which can change with age.
Mass airflow for an automotive engine can also be measured directly, for example, by a sensor injecting ions into the airflow and calculating their transport time to a collecting electrode. Another type of direct measurement mass airflow sensor is the hot wire type wherein a wire is heated to an incandescent state and varies in resistance as a result of the amount of flow cooling the wire. Still another type of direct mass flow sensor is the impact vane configuration. These sensors include a vane or plate with an exposed surface area which is placed in the path of the airflow so that the impingement of the airflow on the surface causes a deflection against a return force which is then measurable as the amount of airflow.
An impact vane sensor is advantageous in that it is a relatively simple mechanical device for the direct measurement of airflow and is relatively accurate while being inexpensive. Further, it does not require a very high potential supply for the generation of ion currents or sensitive electronic circuits that inject current into a hot wire. More importantly, it is relatively insensitive to variations in airflow measurements due to humidity.
The vane type sensors are even more precise because of the recent advance of coupling an air motor to the vane to assist its movement during transient operations. An air motor is a mechanical device generally operating to multiply the mechanical forces of the vane during pressure differentials sensed by two input ports of the air motor. The input ports communicate to the upstream and downstream side of the vane such that if the airflow changes across the vane and thus causes a differential pressure change then the air motor will multiply the forces present and assist the movement of the vane to cancel the change in pressures. This permits the vane to rapidly follow transient operations of the throttling member of the induction tube to provide an even more accurate signal indicative of mass airflow.
One of the more perplexing problems encountered with the vane type sensor is the measurement of airflows near a zero flow. The clearance between the vane and induction tube produce an uncontrolled amount of leakage around the sensor not important at higher airflows. This unmeasured flow and the high frictional forces of most sensors produce an error at low airflows. Moreover, this error can change with the aging of the sensor in a manner difficult to predict. Intricate casting and complex dynamic balancing techniques have been considered to improve the resolution of these sensors at low airflows but are relatively expensive to implement.
It would, therefore, be highly desirable to make this already advantageous device even more favorable for use in the automotive environment by increasing its accuracy without adding to the expense of manufacture of the device. Particularly, if the sensitivity and resolution at low airflow rates could be increased, the device will become more advantageous. This is because automotive applications require highly precise air/fuel ratio metering at low engine speeds and airflow rates because of the characteristics of the engine. Specifically, at idle and low partial throttle conditions, pollution control requires accurate airflow measurement.
Additionally, the vane type airflow meters do exhibit some errors due to frictional losses, transient delays in the movement of the vane even when equipped with air motors, and density effects caused by altitude and temperature. It would, therefore, be highly desirable to compensate a vane air/flow meter for these errors in a fascile manner.